Removing the `Rule` variant from `MentionUri` in #58080 broke
deserialization of any thread saved before the rules-to-skills
migration. Loading one failed with `unknown variant `Rule``, making
those threads inaccessible.

This restores access without bringing back the rules feature:

- Reintroduce a backward-compatible `Rule` mention variant so legacy
threads deserialize again.
- Preserve the original rule `id` through save/load so a thread re-saved
by a newer build still loads on older Zed versions (and synthesize a
valid placeholder when the id is missing).
- Clicking a legacy rule mention now opens the skill the rule was
migrated into, and shows a notification linking the Skills docs. On
remote/collab projects the migrated file lives on the local machine, so
it opens as a read-only buffer. If no matching skill exists, it does
nothing.

Closes https://github.com/zed-industries/zed/issues/58498
Closes AI-371

Release Notes:

- Fixed old threads that mention Rules being inaccessible after Rules
were migrated to Skills